1. Field of the Invention
The present invention generally relates to time division multiplexed interferometric sensors. More specifically, the present invention relates to interrogating interferometric sensors in a manner that improves signal-to-noise ratios.
2. Description of the Related Art
A interferometric sensor system may comprise a transmitter unit that produces an interrogation signal for the interferometric sensors, a sensor network, and a receiver unit that detects the signals from the sensor network. The sensor network may comprise several optical pathways from its input to its output, and some pairs of optical pathways form sensor interferometers. These optical pathways are called sensor pathways. Each sensor interferometer comprises a sensor and lead paths, the parts of the two sensor pathways that are not common define the sensor, while the common parts define the lead paths. In a fiber optic sensor network the lead paths are called lead fibers. The portion of the lead paths between the transmitter unit and a sensor is called the down-lead path and the portion of the lead paths between a sensor and the receiver unit is called the up-lead path. The portion of the lead paths that are common to both the down-lead path and the up-lead path is called the common lead path, or common lead fiber for a fiber optic sensor network. The sensors interferometer can be Michelson interferometers, Mach-Zender interferometers or Fabry-Perot interferometers. The sensor network can be a number of topologies, including a star network, a ladder network, a transmissive serial array, a serial Michelson array or an inline Fabry-Perot sensor array. The different paths through the sensor network may typically be formed by optical waveguides and splitters like optical fibers, optical splitters, circulators, and other waveguide coupled components, or free space optical paths, mirrors, beam splitters and other bulk components. The time delay difference τs between the two sensor pathways is called the imbalance of that sensor, which is typically equal for all sensors. The sensor phase, which is the phase delay difference between the two sensor pathways, can be made sensitive to some physical property that one wants to measure. Thus, information about the physical property can be found by extracting the phase of the interference between the interrogation signal that has propagated the two sensor pathways.
Time-division multiplexing (TDM) of an interferometric sensor network is a form of pulsed interrogation that is achieved by producing light pulses within the transmission unit and transmitting the pulses into the sensor network in one or more pulse transmission time intervals. In between the pulses there may be time intervals without any transmitted light, which are called dark transmission time intervals. Each pulse transmission time interval has typically a length similar to the imbalance of the interrogated sensors. The interrogation signal is made up from a sequence of TDM repetition periods, where each TDM repetition period comprises a sequence of pulse transmission time intervals and dark transmission time intervals. Typically, the TDM repetition periods have equal length and the delay from the start of the TDM repetition periods to the respective pulse and dark transmission time intervals is fixed. A sequence of pulse transmission time intervals that are positioned equally in consecutive TDM repetition periods is called a pulse transmission time slot. Similarly, a sequence of dark transmission time intervals positioned equally in consecutive TDM repetition periods is called a dark transmission time slot. The following description uses transmission time slot as the collective term for pulse transmission time slot and dark transmission time slot. The signal of a transmission time slot is defined by masking out the interrogation signal during the time intervals that define the transmission time slot. The phase or frequency of the optical signal within a transmission time slot is typically varied.
Signals from two pulse transmission time slots are combined at the receiver unit in a receiver time slot after having propagated the two sensor pathways of a sensor interferometer. The interference signal within this receiver time slot includes information about the sensor phase. One or more receiver time slots are associated with the sensor, and the optical signal in at least one receiver time slot is detected, sampled with a sample rate that is equal to or an integer fraction of the TDM repetition rate and processed to extract a demodulated sensor phase as a measure for the sensor. The bandwidth of the demodulated sensor phase signal is less than the receiver Nyquist bandwidth, which is half the sampling rate. Any component of the sensor phase signal above the receiver Nyquist bandwidth is aliased. Thus, the TDM repetition period must therefore be chosen so that aliasing of the sensor phase signal is avoided. TDM of several sensors is typically achieved by having a different delay from the transmission unit to the receiver unit for each of the sensors so that different sensors are associated with different receiver time slots. A receiver time slot may also include information about the sensor phase of more than one sensor, and a set of receiver time slots can be processed to extract information about the individual sensors, as disclosed in O. H. Waagaard, “Method and Apparatus for Reducing Crosstalk Interference in an Inline Fabry-Perot Sensor Array,” U.S. patent application Ser. No. 10/649,588, which is herein incorporated by reference.
A well-known time division multiplexed interrogation technique is the two pulse heterodyne sub-carrier generation technique as disclosed in J. P. Dakin, “An Optical Sensing System,” U.K. patent application number 2,126,820A (filed Jul. 17, 1982). The two pulse heterodyne technique repeatedly transmits two interrogation pulses in two pulse transmission time slots. The phase difference between the first and the second pulse from a TDM period to the next is linearly varied with time to produce a differential frequency shift between the two pulse transmission time slots. The signal from the two pulse transmission time slots that has propagated the two sensor pathways interferes within a receiver time slot. The interference signal comprises a component at a sub-carrier frequency equal to the differential frequency shift. The phase of this sub-carrier provides a measure for the sensor phase.
A well-known interrogation method for continuous wave (cw) interrogation of interferometric sensors is the phase generated carrier technique, disclosed in A. Dandrige, et al., “Homodyne Demodulation Scheme for Fiber Optic Sensors Using Phase Generated Carrier,” IEEE Journal of Quantum Electronics, 18(10):1647-1653, 1982. The phase generated carrier technique is based on a harmonic bias modulation of the phase of the interference signal, for instance, by modulation of the source phase, resulting in a detected interference signal that has signal components at harmonics of the source modulation frequency. The sensor phase can be determined from a combination of the signal components of several harmonics of the source modulation frequency. This technique can also be used in combination with time-division multiplexing, see A. D. Kersey, et al. “Time-division Multiplexing of Interferometric Fiber Sensor Using Passive Phase-generated Carrier Interrogation,” Optics Letters, 12(10):775-777, 1987. The light source may then be pulsed in the same manner as for the two pulse heterodyne sub-carrier generation technique, while the source phase is modulated in the same manner as for the cw phase generated carrier technique. The detector is sampled at the arrival of the reflected pulses, and the sensor phase is calculated from the harmonics of the source modulation frequency.
With one interrogation method specially suited for interrogation of Fabry-Perot sensors, a multiple of interrogation pulses (larger than two) are generated within three or more pulse transmission time slots, see O. H. Waagaard and E. Rønnekleiv, “Multi-pulse Heterodyne Sub-carrier Interrogation of Interferometric Sensors,” U.S. patent application Ser. No. 10/862,123, which is herein incorporated by reference. The phases of the different pulse transmission time slots are modulated at different linear rates. This method improves the signal-to-noise ratio because the multiple reflections generated within the Fabry-Perot cavity do not have to fade out between each pair of interrogation pulses as would be the case for two-pulse interrogation methods.
Unwanted light components that have propagated through other optical pathways from the transmitter unit to the receiver unit other than the two sensor pathways may lead to noise in the demodulated sensor phase or crosstalk from other sensors if these light components overlap with the sensor interference signal within the receiver time slots. For each interrogated sensor, the noise contributing pathways are define as all these optical pathways from the transmitter unit to the receiver unit apart from sensor pathways. Since the light components that have propagated through a noise contributing pathway have significantly lower amplitude than the light components that have propagated through the sensor pathways, the noise and crosstalk caused by these unwanted light components can be significantly reduced if the interference between the unwanted light components and the interference signal from the interrogated sensor can be suppressed.
A noise contributing pathways may arise due to discrete reflectors such as reflectors of other sensors, circulators, couplers, connectors, etc., or due to distributed reflectors such as Rayleigh scattering. If TDM is combined with wavelength division multiplexing (WDM), wavelength selective components such as fiber Bragg gratings (FBGs) or WDM-splitters have limited sideband suppression. Thus, the interrogation signal within a certain WDM-channel may propagate optical pathways belonging to a sensor of a different WDM-channel. The delay of a noise contributing pathway may be such that a pulse that has propagated the noise contributing pathway is received by the receiver unit within a receiver time slot that is used to demodulate the sensor phase. This is the case if the difference in delay between the noise contributing pathway and one of the sensor pathways is equal to the delay between two pulse transmission time intervals. If the common lead path to the sensor is longer than the TDM repetition period, such noise contributing pathways may arise due to Rayleigh reflection along the common lead path. The points along the common lead path that give rise to such noise contributing pathways are called collision points.
A noise contributing pathway can also be a sensor pathway of other time-division multiplexed sensors within the same WDM-channel. If there is no light within the dark transmission time slots, these pathways do not contribute with noise and crosstalk on the interrogated sensor since the optical signal from another time-division multiplexed sensor appears in another receiver time slot. However, limited on/off extinction of the interrogation pulses, for instance, due to light leakage during the dark transmission time slots, may give rise to other light components that may interfere with the interference signal of the interrogated sensor. Such unwanted interference may also lead to unwanted demodulated noise and crosstalk. One proposed method for suppression of this interference includes applying a large phase generated carrier modulation with frequency fpgc to a lithium niobate phase modulator during the dark transmission time slots, and thereby moving the signal components due to interference between one of the pulses and leakage light to multiples of fpgc, see D. Hall and J. Bunn, “Noise Suppression Apparatus and Method for Time Division Multiplexed Fiber Optic Sensor Arrays,” U.S. Pat. No. 5,917,597, 1999. However, the amount of suppression of this interference depends on the time delay between the generated pulse and the leakage light, and there is no suppression when the time delay is 1/fpgc. Also, a very large voltage signal has to be applied to the phase modulator, which makes this method impractical.
Therefore, there exists a need in the art for a method that reduces the sensitivity to the interference with unwanted light components reflected from other parts of a TDM sensor network than the interrogated sensor.